1. Technical Field
The present invention relates to a structure of a casing of a small stepping motor, and, more particularly, to a structure of a casing of a small stepping motor having an outer diameter of 8 mm or less, in which a plurality of yoke teeth is integrally formed on the inner surface of the casing, and the thickness of the outer wall of the casing is different from the thickness of each of the yoke teeth, thus reducing a production cost and improving a process.
2. Description of the Related Art
Generally, an optical pickup apparatus, such as office automation (OA) appliances, optical disk drives (ODDs), hard disk drives (HDDs) or the like, is an apparatus for irradiating data recorded in an optical disk with a laser beam using an optical pickup actuator to decipher the data and then transmitting the deciphered data to a display device. Such an optical pickup apparatus is provided with a stepping motor, and the stepping motor is used to pickup the optical pickup apparatus while transporting it from a base by a predetermined distance.
The stepping motor includes a rotor equipped with a magnet and a stator spaced apart from the rotor by a predetermined distance, and is configured such that the rotor is rotated by external control signals applied to the stator.
As shown in FIG. 1, a general stepping motor includes a rotor 200, a pair of stators 100 and 100a spaced apart from the rotor 200, and a casing 300 covering the pair of stators 100 and 100a to protect them.
Here, the rotor 200 includes a shaft 210, a magnet 220 provided on the outer surface of the shaft 210, and a spacer 230 disposed between the shaft 210 and the magnet 220 such that the shaft 210 is positioned at the center of the rotor 200, and is disposed in the space defined by the inner surface of the pair of stators 100 and 100a. 
The pair of stators 100 and 100a include a first stator 100 and a second stator 100a. Each of the first stator 100 and the second stator 100a includes a bobbin 110 wound with a coil 110a and yokes 120 provided with a plurality of yoke teeth 120a supporting the inner surface of the bobbin 110.
Each of the first stator 100 and the second stator 100a include two yokes 120 opposed to each other, and the yoke teeth 120a of the two yokes 120 are alternately disposed.
The magnetic flux of the magnet 220 passes through the yoke teeth 120a of the first stator 100 alternately disposed, and the yoke teeth 120 of the second stator 100a alternately disposed. The stepping motor is configured such that it is rotated by external control signals applied to the stators 100 and 100a. In this case, the thickness of the yoke teeth 120a is closely related to the magnetic flux density of the magnet 220.
However, the conventional stepping motor is problematic in that it has four yokes 120, each being provided with a plurality of yoke teeth 120a, so that a large number of parts are required, thereby increasing the production costs thereof.
In order to solve the above problem, a stepping motor which can reduce the number of yokes by integrally forming yoke teeth on the inner surface of a casing has been developed. However, only the idea that the number of yokes can be reduced by integrally forming yoke teeth on the inner surface of the casing is considered, so that the thickness of the outer wall of the casing becomes equal to that of the yoke teeth, with the result that it is difficult to decrease the outer diameter of a stepping motor having an outer diameter of 8 mm or less. Therefore, there is a problem in that, when the thickness of the outer wall of the casing is decreased in order to decrease the thickness of this stepping motor, the thickness of the yoke teeth is also decreased, so that the stepping motor cannot exhibit normal torque characteristics.